UCLA scientists discuss implications of increased light in Milky Way black hole
(Andrea Grigsby/Illustrations Director)
By Shruti Iyer
Oct. 7, 2019 12:44 a.m.
An influx of light in the center of the Milky Way galaxy may lead to further insight into black holes, UCLA experts suggest.
Scientists from the UCLA Galactic Center Group discovered May 13 that the supermassive black hole, Sagittarius A*, at the center of our galaxy had gotten unusually bright. The results were published in The Astrophysical Journal Letters Sept. 11.
“Normally, when one thinks about a black hole, one doesn’t expect it to emit light because the definition of black hole is that nothing, not even light can escape it,” said Tuan Do, a UCLA research scientist at the Galaxy Center Group.
However, as material gravitates toward the edge of the black hole, it heats up and warms up the surrounding gases. Therefore, the light detected is emitted by gases that are about to fall into the black hole, not light directly from it, Do said.
Do, who is the study’s first author, said the location of Sagittarius A* became twice as bright as anything else seen in the center of the galaxy in the last 20 years.
“Over a period of two hours, (the region’s) brightness changed by a factor of 75 or so,” Do said. “It got brighter than all of the stars in this region. That’s why it was so unusual.”
Mark Morris, a UCLA astronomy and astrophysics professor, said the study’s conclusion was that a dramatic event had taken place as the black hole was “eating” something or “eating” at a much greater rate than normal. Morris said that this process of “eating” is actually the accretion of matter by the black hole.
It was also possible that the matter, which was about to fall into the black hole, underwent a strong magnetic interaction with the black hole’s magnetic field. This generated huge amounts of energy that translated into an increase in brightness in the infrared radiation it produced, he said.
Andrea Ghez, director of the Galactic Center Group and a UCLA physics and astronomy professor, said that one of the things researchers had to be careful about was if the variation was fully understood by scientists.
One idea was that the flare-up was a rare extreme event, similar to big earthquakes, Ghez said.
“This is like a big Thanksgiving dinner – it happens very rarely,” Ghez said “So you weren’t ready to anticipate it.”
S0-2, the brightest star near the black hole, was making its closest approach to Sagittarius A* around the time the black hole was very bright. This could have also contributed to the flare, as it could have been the result of an object that went by the black hole and disturbed the environment, Ghez said.
Morris said the group’s initial idea 20 years ago had been to observe the stars and their orbits around the black hole in order to study it, but then quickly discovered the black hole itself was emitting variable infrared radiation.
“We were paying close attention to what the black hole was doing, putting on its light show for us,” he said.
Zhuo Chen, a physics and astronomy graduate student, said she had been working on studying the fluctuating brightness of the supermassive black hole for three years since she entered UCLA. The luminosity – the total energy emitted by the area surrounding the black hole over one unit of time – in the infrared radiation emitted by Sagittarius A* varies based on how much gas and dust fall into it, Chen said.
“The most commonly accepted model for the (behavior of) Sagittarius A* is the red noise process,” Chen added.
Red noise is similar to white noise – the static that can be heard in radios. Infrared radiation released by Sagittarius A* tends to increase until the radiation’s amplitude reaches a constant level, Chen said. This constant level is the black hole’s red noise. Other infrared emissions are compared to this quantity, Chen added.
Do said the group has been using the Keck Observatory in Hawaii to observe the center of the Milky Way for about 20 years. They use infrared light to observe the region, Do added.
“We use (this historical) data to learn about the environment at the center of our galaxy,” he said. “(This includes) the black hole and the stars in the region.”
Ghez said the biggest problem in measuring data is the atmosphere, which can blur the image of astronomical sources.
The Keck Observatory accounts for this by using an adaptive optics system, which has a flexible mirror that can be modified to compensate for the distorting effects of the atmosphere, Ghez said, who is the study’s co-senior author.
“The atmosphere would be a circus fun house mirror that’s moving all the time,” Ghez said. “So it makes (the image) look all funny, but funny in a different way, every time you look. And so what we’re trying to do is put in a second mirror that takes on the opposite shade, so that (the image) looks flat again.”
Morris said scientists now have more reason to continue observing the center of the Milky Way. He added that further observation is required to obtain a clearer idea of what happened. Therefore, data measured in other wavelengths, such as radio and X-ray, must be taken and compared to the infrared data.
“We had a nice, clean, clean statistical description of the fluctuations in the brightness of the black hole,” Morris said. “And this event violated our nice, clean statistical description. And so that tells us that our description isn’t complete.”